1. Field of the Invention
This invention relates to an image forming apparatus and an image forming method.
2. Description of the Related Art
Conventionally as a printing method for printing characters, an image and the like on a print medium such as a print paper or film, it is known that there is an Inkjet printing method for making ink as a print agent (color material) adhere on the print medium to form an image on the print medium.
Ink including pigment as a color material is widely used in an inkjet printing apparatus according to the inkjet printing method. The pigment ink has the feature that the color material as a solid content component tends to more easily deposit on a surface of the print medium, as compared to the dye ink. FIG. 1A and FIG. 1B illustrate schematic diagrams of the pigment color material which has deposited on the print medium.
A printing form of the inkjet printing apparatus includes the serial type of a form. In the serial type of the inkjet printing apparatus, ink is ejected from the print head while a main scan and a sub scan are alternately repeated, thus sequentially forming an image on the print medium. Here, printing of the main scan is performed such that a carriage mounting the print head thereon moves over the print medium in the main scan direction for printing. On the other hand, printing of the sub scan is performed along with the change of a printing position by carrying the print medium in a direction perpendicular to the main scan direction by a predetermined amount. In this case, a width of a region to be printed by the main scan of one time is defined by a head length of a plurality of ink ejection ports provided in the print head.
For further enhancing a quality level of an image, a multi-scan method is adopted. The multi-scan method performs printing of main scans by N number of times (N≧2) onto an image region printable by printing of the main scan of one time. Adoption of the multi-scan method brings in the effect that, by carrying the print medium by a predetermined amount in printing of each main scan, variations in printing by each print element and variations in a sub scan amount are dispersed to smooth an entire image. Therefore in the serial type of the inkjet printing apparatus, the multi-scan method is advantageously adopted at present.
The effect of the multi-scan method can become the larger as the more numbers of multi-scans are set, but on the other hand, may lead to an increase of operation time in printing. In recent years, an inkjet printing apparatus has some recording modes respectively set with the number of multi-scan, and a user can select an appropriate mode in accordance with the type or the application of the print image.
In addition, the printing of the main scan in the multi-scan method includes two methods, that is, one-way printing in which the printing is performed only in the forward direction and bidirectional printing in which the printing is performed alternately in both of the forward direction and the backward direction. In the bidirectional printing, an image region formed by the scan in the forward direction and an image region formed by the scan in the backward direction are alternately generated in each width of the regions printed by the printing main scan of one time. Without mentioning, a printing speed in the bidirectional printing is faster than in the one-way printing.
However, there are some cases where “band irregularities” generates in the bidirectional printing, which does not generate in the one-way printing. “The band irregularity” is the problem occurring because of a difference in an arrangement of ink colors to be printed between the image region formed by the scan in the forward direction and the image region formed by the scan in the backward direction. That is, even if the printing is performed according to the same data, there occurs a difference as clear as to be visually confirmable between a color of the image printed in the forward direction and a color of the image printed in the backward direction. Particularly in a case of using the pigment ink, since the color material has the properties of tending to accumulate on a surface of the print medium, the arrangement of the ink colors to be printed has a great impact on the image quality. As a result, in some cases the ink irregularity is noticeable.
Hereinafter, it will be explained with reference to FIGS. 2A and 2B that the arrangement of the ink colors to be printed is different depending on the scan direction. Here, there will be explained an example where two kinds of cyan ink and magenta ink are used to cause both of the inks to land on a predetermined position of the print medium one by one.
FIG. 2A illustrates printing in the forward direction and FIG. 2B illustrates printing in the backward direction. A head 201 is provided with a cyan nozzle 202 used for printing cyan ink and a magenta nozzle 203 used for printing magenta ink. In a case where the forward direction is defined as a front side in the printing main scan direction, the cyan nozzle 202 and the magenta nozzle 203 are assumed to be arranged in order from the front side. As shown in FIG. 2A, in a case of the forward scan, since the nozzle of the cyan ink performs ejection ahead of the nozzle of the magenta ink, the cyan ink lands on the print medium ahead of the magenta ink (cyan dot 204), and the magenta ink lands on the cyan ink (magenta dot 205). On the other hand, as shown in FIG. 2B, in the backward scan, since the nozzle of the magenta ink performs ejection ahead of the nozzle of the cyan ink, the magenta ink lands on the print medium ahead of the cyan ink (magenta dot 205), and the cyan ink lands on the magenta ink (cyan dot 204). As described above, since the landing-on order of the ink colors to be printed differs between the forward scan and backward scan, the arrangement of the ink colors to be printed results in being different depending on the direction of the printing scan.
Some measures using mask patterns against the band irregularity are disclosed in public. It should be noted that the mask pattern is used for image data in multi-scan printing for each printing main scan (also called as a pass).
For example, it is proposed a method “in which in a plurality of thinning mask patterns corresponding to colors differing with each other, a pixel arrangement of at least one of the thinning mask patterns is different from a pixel arrangement of the other thinning mask pattern” (for example, Japanese Patent No. 3200143). In the same printing scan, the printing is performed in positions different with each other between respective colors, thus reducing a difference in color between the forward printing and the backward printing.
In addition, it is proposed a method in which a mask pattern is provided to correspond to each of a plurality of blocks in a fixed manner and a mutual interpolation relation is maintained between the blocks, which is applied in the same way between a first print head and a second print head (for example, Japanese Patent No. 3236034). According to this structure, by fixing the mask pattern to the print head, band irregularities due to a deviation in a printing ratio between the respective printing scans generated by an arrangement state between the mask pattern and the image data can be reduced.
However, the method described in Japanese Patent No. 3200143 or Japanese Patent No. 3236034 has no system for changing the processing corresponding to the image, and therefore, for example, in a case of an input image where the band irregularity tends to be noticeable, it is hard to say that the band irregularity can be sufficiently reduced.
Therefore an object of the present invention is to reduce band irregularities regardless of an input image.
Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings.